The present invention relates to thermal interfaces. Specifically, the present invention is directed to thermally conductive media ideally suited for systems which cycle over a range of temperatures.
Conventional thermal interface media reduce the temperature gradient between two different surfaces in close proximity with one another. The surfaces are typically mating surfaces. Conventional interface media are positioned in the gaps or voids between the two surfaces so that the thermal resistance is lowered, thereby allowing the heat to flow away from the hotter surface. The efficient flow of heat may be impeded if any gaps or voids remain at the interface surfaces. Therefore, not only must the interface medium be thermally conductive, but it must also compensate for certain manufacturing tolerances inherent in the interface.
Thermal paste is a common interface medium that provides heat transfer as well as conformal properties. A typical thermal paste may contain a mixture of zinc oxide in mineral oil. Thermal conductivity is achieved by peculating oxide particles in a low conductivity oil matrix.
U.S. Pat. No. 5,198,189 to Booth et al. discloses, in pertinent part, a liquid metal matrix thermal paste in which non-reacting thermally conductive particles are dispersed in a metal matrix having a low melting temperature. The particles are silicon, molybdenum, tungsten or other materials which do not react with gallium at temperatures below approximately 100.degree. C. The preferred liquid metals are described as gallium and indium eutectic, gallium and tin eutectic, and gallium, indium and tin ternary eutectic. The liquid metal matrix thermal paste is described as being used for cooling high power dissipation components in conjunction with a conventional fluid cooling system. The paste may be cleaned from surfaces by using metal wool containing tin or copper filaments.
The use of thermally conductive pastes, however, is problematic. The paste must be applied with precision. If applied in incorrect quantities, e.g., if too thick, the heat transfer performance degrades. In addition, unwanted material, such as machining chips, tends to collect in the paste so that even larger gaps are produced, which can also reduce heat transfer performance. This problem is exacerbated by the difficulty of removing the paste without leaving a residue.
Thermally conductive gaskets overcome the problem associated with removing the paste from the interface. U.S. Pat. No. 4,776,602 to Paul E. Gallo discloses a conventional thermally conductive gasket that includes a metallic core with an upper and lower face. The core is fabricated from tin plated stainless or low carbon steel. A thermally conductive expandable graphite material contacts with the upper face. A pair of compressible non-asbestos facing layers are disposed on opposing sides of the core and comprise of clay, rubber, and aramid fibers. Tangs are formed in the core to clinch together all the layers in the device.
U.S. Pat. No. 5,137,283 to Giarusso et al. discloses, in pertinent part, a thermally conductive gasket formed by encapsulating a thermally conductive low melting temperature material in a plastic skin. The low melting temperature material conforms to the shape of the interface so as to completely fill the voids once heated above the aforementioned melting temperature. This provides a gasket which is easily applied and removed from the interface, thereby affording a re-usable gasket.
U.S. Pat. No. 5,459,352 to Layton et al. discloses, in pertinent part, a liquid metal aluminum/copper thermal conductor to provide a path for conducting heat from a chip to a fluid medium. In one embodiment, the thermal conductor is described as being formed from a fibrous metal body coated with a liquid metal alloy. The liquid metal alloy may include gallium, indium, selenium, zinc, or mercury. However, conventional thermal gaskets have limited compressibility necessitating increased thickness to fill voids which reduces the heat transfer qualities of the same.
What is needed, therefore, is a highly conformal thermally conductive medium that is reusable.